Previous studies done at UNC have shown that 18-35 year old individuals with cystic fibrosis have difficulty in absorbing calcium and vitamin D from their diets when compared with normal individuals of the same age. Failure to absorb dietary calcium is likely to be a cause of weak bones, or osteoporosis, in people with cystic fibrosis. Vitamin D is important in protecting against osteoporosis because it promotes calcium absorption from the gastrointestinal tract. Our purpose in the present study is to learn whether the administration of an activated form of vitamin D over a period of time can improve the absorption of calcium in individuals with cystic fibrosis as well as in normal subjects.